Inverse shape replication method for forming metal matrix composite bodies and products produced therefrom

ABSTRACT

The present invention relates to the formaton of metal matrix composite bodies by a spontaneous infiltration technique, and novel metal matrix composite bodies produced according to the method. An ingot of matrix metal is surrounded by a permeable mass of filler matrial. An infiltration enhancer and/or an infiltration enhancer precursor and/or an infiltrating atmosphere are also in communication with the filler material, at least at some point during the process, which permit the shaped ingot of matrix metal, when made molten, to spontaneously infiltrate the surrounding permeable mass of filler material. After the spontaneous infiltration, a metal matrix composite body is produced having therein a cavity which substantially corresponds in shape to the original ingot of matrix metal.

FIELD OF THE INVENTION

The present invention relates to the formation of metal matrix compositebodies by a spontaneous infiltration technique, and novel metal matrixcomposite bodies produced according to the method. Particularly, aningot of matrix metal is shaped into a pattern which is substantiallycomplementary in shape to a cavity which is to be formed in a metalmatrix composite body. The shaped ingot of matrix metal is surrounded bya permeable mass of filler material, which under the process conditionsmay be conformable to the shaped ingot of matrix metal. An infiltrationenhancer and/or an infiltration enhancer precursor and/or aninfiltration atmosphere are also in communication with the fillermaterial, at least at some point during the process, which permit theshaped ingot of matrix metal, when made molten, to spontaneouslyinfiltrate the surrounding permeable mass of filler material. After suchspontaneous infiltration, a metal matrix composite body is producedhaving therein a cavity which substantially corresponds in shape to theoriginal shaped ingot of matrix metal.

BACKGROUND OF THE INVENTION

Composite products comprising a metal matrix and a strengthening orreinforcing phase such as ceramic particulates, whiskers, fibers or thelike, show great promise for a variety of applications because theycombine some of the stiffness and wear resistance of the reinforcingphase with the ductility and toughness of the metal matrix. Generally, ametal matrix composite will show an improvement in such properties asstrength, stiffness, contact wear resistance, and elevated temperaturestrength retention relative to the matrix metal in monolithic form, butthe degree to which any given property may be improved depends largelyon the specific constituents, their volume or weight fraction, and howthey are processed in forming the composite. In some instances, thecomposite also may be lighter in weight than the matrix metal per se.Aluminum matrix composites reinforced with ceramis such as siliconcarbide in particulate, platelet, or whisker form, for example, are ofinterest because of their higher stiffness, wear resistance and hightemperature strength relative to aluminum.

Various metallurgical processes have been described for the fabricationof aluminum matrix composites, including methods based on powdermetallurgy techniques and liquid-metal infiltration techniques whichmake use of pressure casting, vacuum casting, stirring, and wettingagents. With powder metallurgy techniques, the metal in the form of apowder and the reinforcing material in the form of a powder, whiskers,chopped fibers, etc., are admixed and then either cold-pressed andsintered, or hot-pressed. The maximum ceramic volume fraction in siliconcarbide reinforced aluminum matrix composites produced by this methodhas been reported to be about 25 volume percent in the case of whiskers,and about 40 volume percent in the case of particulates.

The production of metal matrix composites by powder metallurgytechniques utilizing conventional processes imposes certain limitationswith respect to the characteristics of the products attainable. Thevolume fraction of the ceramic phase in the composite is limitedtypically, in the case of particulates, to about 40 percent. Also, thepressing operation poses a limit on the practical size attainable. Onlyrelatively simple product shapes are possible without subsequentprocessing (e.g., forming or machining) or without resorting to complexpresses. Also, nonuniform shrinkage during sintering can occur, as wellas nonuniformity of microstructure due to segregation in the compactsand grain growth.

U.S. Pat. No. 3,970,136, granted July 20, 1976, to J. C. Cannell et al.,describes a process for forming a metal matrix composite incorporating afibrous reinforcement, e.g. silicon carbide or alumina whiskers, havinga predetermined pattern of fiber orientation. The composite is made byplacing parallel mats or felts of coplanar fibers in a mold with areservoir of molten matrix metal, e.g., aluminum, between at least someof the mats, and applying pressure to force molten metal to penetratethe mats and surround the oriented fibers. Molten metal may be pouredonto the stack of mats while being forced under pressure to flow betweenthe mats. Loadings of up to about 50% by volume of reinforcing fibers inthe composite have been reported.

The above-described infiltration process, in view of its dependence onoutside pressure to force the molten matrix metal through the stack offibrous mats, is subject to the vagaries of pressure-induced flowprocesses, i.e., possible non-uniformity of matrix formation, porosity,etc. Non-uniformity of properties is possible even though molten metalmay be introduced at a multiplicity of sites within the fibrous array.Consequently, complicated mat/reservoir arrays and flow pathways need tobe provided to achieve adequate and uniform penetration of the stack offiber mats. Also, the aforesaid pressure-infiltration method allows foronly a relatively low reinforcement to matrix volume fraction to beachieved because of the difficulty inherent in infiltrating a large matvolume. Still further, molds are required to contain the molten metalunder pressure, which adds to the expense of the process. Finally, theaforesaid process, limited to infiltrating aligned particles or fibers,is not directed to formation of aluminum metal matrix compositesreinforced with materials in the form of randomly oriented particles,whiskers or fibers.

In the fabrication of aluminum matrix-alumina filled composites,aluminum does not readily wet alumina, thereby making it difficult toform a coherent product. Various solutions to this problem have beensuggested. One such approach is to coat the alumina with a metal (e.g.,nickel or tungsten), which is then hot-pressed along with the aluminum.In another technique, the aluminum is alloyed with lithium, and thealumina may be coated with silica. However, these composites exhibitvariations in properties, or the coatings can degrade the filler, or thematrix contains lithium which can affect the matrix properties.

U.S. Pat. No. 4,232,091 to R. W. Grimshaw et al., overcomes certaindifficulties in the art which are encountered in the production ofaluminum matrix-alumina composites. This patent describes applyingpressures of 75-375 kg/cm² to force molten aluminum (or molten aluminumalloy) into a fibrous or whisker mat of alumina which has been preheatedto 700° to 1050° C. The maximum volume ratio of alumina to metal in theresulting solid casting was 0.25/1. Because of its dependency on outsideforce to accomplish infiltration, this process is subject to many of thesame deficiencies as that of Cannell et al.

European Patent Application Publication No. 115,742 describes makingaluminum-alumina composites, especially useful as electrolytic cellcomponents, by filling the voids of a preformed alumina matrix withmolten aluminum. The application emphasizes the non-wettability ofalumina by aluminum, and therefore various techniques are employed towet the alumina throughout the preform. For example, the alumina iscoated with a wetting agent of a diboride of titanium, zirconium,hafnium, or niobium, or with a metal, i.e., lithium, magnesium, calcium,titanium, chromium, iron, cobalt, nickel, zirconium, or hafnium. Inertatmospheres, such as argon, are employed to facilitate wetting. Thisreference also shows applying pressure to cause molten aluminum topenetrate an uncoated matrix. In this aspect, infiltration isaccomplished by evacuating the pores and then applying pressure to themolten aluminum in an inert atmosphere, e.g., argon. Alternatively, thepreform can be infiltrated by vapor-phase aluminum deposition to wet thesurface prior to filling the voids by infiltration with molten aluminum.To assure retention of the aluminum in the pores of the preform, heattreatment, e.g., at 1400° to 1800° C., in either a vacuum or in argon isrequired. Otherwise, either exposure of the pressure infiltratedmaterial to gas or removal of the infiltration pressure will cause lossof aluminum from the body.

The use of wetting agents to effect infiltration of an alumina componentin an electrolytic cell with molten metal is also shown in EuropeanPatent Application Publication No. 94353. This publication describesproduction of aluminum by electrowinning with a cell having a cathodiccurrent feeder as a cell liner or substrate. In order to protect thissubstrate from molten cryolite, a thin coating of a mixture of a wettingagent and solubility suppressor is applied to the alumina substrateprior to start-up of the cell or while immersed in the molten aluminumproduced by the electrolytic process. Wetting agents disclosed aretitanium, zirconium, hafnium, silicon, magnesium, vanadium, chromium,niobium, or calcium, and titanium is stated as the preferred agent.Compounds of boron, carbon and nitrogen are described as being useful insuppressing the solubility of the wetting agents in molten aluminum. Thereference, however, does not suggest the production of metal matrixcomposites, nor does it suggest the formation of such a composite in,for example, a nitrogen atmosphere.

In addition to application of pressure and wetting agents, it has beendisclosed that an applied vacuum will aid the penetration of moltenaluminum into a porous ceramic compact. For example, U.S. Pat. No.3,718,441, granted Feb. 27, 1973, to R. L. Landingham, reportsinfiltration of a ceramic compact (e.g., boron carbide, alumina andberyllia) with either molten aluminum, beryllium, magnesium, titanium,vanadium, nickel or chromium under a vacuum of less than 10⁻⁶ torr. Avacuum of 10⁻² to 10⁻⁶ torr resulted in poor wetting of the ceramic bythe molten metal to the extent that the metal did not flow freely intothe ceramic void spaces. However, wetting was said to have improved whenthe vacuum was reduced to less than 10⁻⁶ torr.

U.S. Pat. No. 3,864,154, granted Feb. 4, 1975, to G. E. Gazza et al.,also shows the use of vacuum to achieve infiltration. This patentdescribes loading a cold-pressed compact of AlB₁₂ powder onto a bed ofcold-pressed aluminum powder. Additional aluminum was then positioned ontop of the AlB₁₂ powder compact. The crucible, loaded with the AlB₁₂compact "sandwiched" between the layers of aluminum powder, was placedin a vacuum furnace. The furnace was evacuated to approximately 10⁻⁵torr to permit outgassing. The temperature was subsequently raised to1100° C. and maintained for a period of 3 hours. At these conditions,the molten aluminum penetrated the porous AlB₁₂ compact.

U.S. Pat. No. 3,364,976, granted Jan. 23, 1968, to John N. Reding etal., discloses the concept of creating a self-generated vacuum in a bodyto enhance penetration of a molten metal into the body. Specifically, itis disclosed that a body, e.g., a graphite mold, a steel mold, or aporous refractory material, is entirely submerged in a molten metal. Inthe case of a mold, the mold cavity, which is filled with a gas reactivewith the metal, communicates with the externally located molten metalthrough at least one orifice in the mold. When the mold is immersed intothe melt, filling of the cavity occurs as the self-generated vacuum isproduced from the reaction between the gas in the cavity and the moltenmetal. Particularly, the vacuum is a result of the formation of a solidoxidized form of the metal. Thus, Reding et al. discloses that it isessential to induce a reaction between gas in the cavity and the moltenmetal. However, utilizing a mold to create a vacuum may be undesirablebecause of the inherent limitations associated with use of a mold. Moldsmust first be machined into a particular shape; then finished, machinedto produce an acceptable casting surface on the mold; then assembledprior to their use; then disassembled after their use to remove the castpiece therefrom; and thereafter reclaim the mold, which most likelywould include refinishing surfaces of the mold or discarding the mold ifit is no longer acceptable for use. Machining of a mold into a complexshape can be very costly and time-consuming. Moreover, removal of aformed piece from a complex-shaped mold can also be difficult (i.e.,cast pieces having a complex shape could be broken when removed from themold). Still further, while there is a suggestion that a porousrefractory material can be immersed directly in a molten metal withoutthe need for a mold, the refractory material would have to be anintegral piece because there is no provision for infiltrating a loose orseparated porous material absent the use of a container mold (i.e., itis generally believed that the particulate material would typicallydisassociate or float apart when placed in a molten metal). Stillfurther, if it was desired to infiltrate a particulate material orloosely formed preform, precautions should be taken so that theinfiltrating metal does not displace at least portions of theparticulate or preform resulting in a non-homogeneous microstructure.

Accordingly, there has been a long felt need for a simple and reliableprocess to produce shaped metal matrix composites which does not relyupon the use of applied pressure or vacuum (whether externally appliedor internally created), or damaging wetting agents to create a metalmatrix embedding another material such as a ceramic material. Moreover,there has been a long felt need to minimize the amount of finalmachining operations needed to produce a metal matrix composite body.The present invention satisfies these needs by providing a spontaneousinfiltration mechanism for infiltrating a material (e.g., a ceramicmaterial), which is formed into a preform, with molten matrix metal(e.g., aluminum) in the presence of an infiltrating atmosphere (e.g.,nitrogen) under normal atmospheric pressures so long as an infiltrationenhancer is present at least at some point during the process.

DESCRIPTION OF COMMONLY OWNED U.S. PATENT APPLICATIONS

The subject matter of this application is related to that of severalother copending and co-owned patent applications. Particularly, theseother copending patent applications describe novel methods for makingmetal matrix composite materials (hereinafter sometimes referred to as"Commonly Owned Metal Matrix Patent Applications").

A novel method of making a metal matrix composite material is disclosedin Commonly Owned U.S. patent application Ser. No. 049,171, filed May13, 1987, in the names of White et al., and entitled "Metal MatrixComposites", now allowed in the United States. According to the methodof the White et al. invention, a metal matrix composite is produced byinfiltrating a permeable mass of filler material (e.g., a ceramic or aceramic-coated material) with molten aluminum containing at least about1 percent by weight magnesium, and preferably at least about 3 percentby weight magnesium. Infiltration occurs spontaneously without theapplication of external pressure or vacuum. A supply of the molten metalalloy is contacted with the mass of filler material at a temperature ofat least about 675° C. in the presence of a gas comprising from about 10to 100 percent, and preferably at least about 50 percent, nitrogen byvolume, and a remainder of the gas, if any, being a nonoxidizing gas,e.g., argon. Under these conditions, the molten aluminum alloyinfiltrates the ceramic mass under normal atmospheric pressures to forman aluminum (or aluminum alloy) matrix composite. When the desiredamount of filler material has been infiltrated with the molten aluminumalloy, the temperature is lowered to solidify the alloy, thereby forminga solid metal matrix structure that embeds the reinforcing fillermaterial. Usually, and preferably, the supply of molten alloy deliveredwill be sufficient to permit the infiltration to proceed essentially tothe boundaries of the mass of filler material. The amount of fillermaterial in the aluminum matrix composites produced according to theWhite et al. invention may be exceedingly high. In this respect, fillerto alloy volumetric ratios of greater than 1:1 may be achieved.

Under the process conditions in the aforesaid White et al. invention,aluminum nitride can form as a discontinuous phase dispersed throughoutthe aluminum matrix. The amount of nitride in the aluminum matrix mayvary depending on such factors as temperature, alloy composition, gascomposition and filler material. Thus, by controlling one or more suchfactors in the system, it is possible to tailor certain properties ofthe composite. For some end use applications, however, it may bedesirable that the composite contain little or substantially no aluminumnitride.

It has been observed that higher temperatures favor infiltration butrender the process more conducive to nitride formation. The White et al.invention permits a choice of a balance between infiltration kineticsand nitride formation.

An example of suitable barrier means for use with metal matrix compositeformation is described in Commonly Owned and Copending U.S. patentapplication Ser. No. 141,642, filed Jan. 7, 1988, in the names ofMichael K. Aghajanian et al., and entitled "Method of Making MetalMatrix Composite with the use of a Barrier". According to the method ofthis Aghajanian et al. invention, a barrier means (e.g., particulatetitanium diboride or a graphite material such as a flexible graphitetape product sold by Union Carbide under the trade name Grafoil®) isdisposed on a defined surface boundary of a filler material and matrixalloy infiltrates up to the boundary defined by the barrier means. Thebarrier means is used to inhibit, prevent, or terminate infiltration ofthe molten alloy, thereby providing net, or near net, shapes in theresultant metal matrix composite. Accordingly, the formed metal matrixcomposite bodies have an outer shape which substantially corresponds tothe inner shape of the barrier means.

The method of U.S. patent application Ser. No. 049,171 was improved uponby Commonly Owned and Copending U.S. patent application Ser. No.168,284, filed Mar. 15, 1988, in the names of Michael K. Aghajanian andMarc S. Newkirk and entitled "Metal Matrix Composites and Techniques forMaking the Same." In accordance with the methods disclosed in this U.S.patent application, a matrix metal alloy is present as a first source ofmetal and as a reservoir of matrix metal alloy which communicates withthe first source of molten metal due to, for example, gravity flow.Particularly, under the conditions described in this patent application,the first source of molten matrix alloy begins to infiltrate the mass offiller material under normal atmospheric pressures and thus begins theformation of a metal matrix composite. The first source of molten matrixmetal alloy is consumed during its infiltration into the mass of fillermaterial and, if desired, can be replenished, preferably by a continuousmeans, from the reservoir of molten matrix metal as the spontaneousinfiltration continues. When a desired amount of permeable filler hasbeen spontaneously infiltrated by the molten matrix alloy, thetemperature is lowered to solidify the alloy, thereby forming a solidmetal matrix structure that embeds the reinforcing filler material. Itshould be understood that the use of a reservoir of metal is simply oneembodiment of the invention described in this patent application and itis not necessary to combine the reservoir embodiment with each of thealternate embodiments of the invention disclosed therein, some of whichcould also be beneficial to use in combination with the presentinvention.

The reservoir of metal can be present in an amount such that it providesfor a sufficient amount of metal to infiltrate the permeable mass offiller material to a predetermined extent. Alternatively, an optionalbarrier means can contact the permeable mass of filler on at least oneside thereof to define a surface boundary.

Moreover, while the supply of molten matrix alloy delivered should be atleast sufficient to permit spontaneous infiltration to proceedessentially to the boundaries (e.g., barriers) of the permeable mass offiller material, the amount of alloy present in the reservoir couldexceed such sufficient amount so that not only will there be asufficient amount of alloy for complete infiltration, but excess moltenmetal alloy could remain and be attached to the metal matrix compositebody. Thus, when excess molten alloy is present, the resulting body willbe a complex composite body (e.g., a macrocomposite), wherein aninfiltrated ceramic body having a metal matrix therein will be directlybonded to excess metal remaining in the reservoir.

Each of the above-discussed Commonly Owned Metal Matrix PatentApplications describes methods for the production of metal matrixcomposite bodies and novel metal matrix composite bodies which areproduced therefrom. The entire disclosures of all of the foregoingCommonly Owned Metal Matrix Patent Applications are expresslyincorporated herein by reference.

The subject matter of this application is also related to anothercopending and co-owned patent application relating to the formation of anovel ceramic matrix composite material (hereinafter sometimes referredto as "Commonly Owned Ceramic Matrix Patent Application").

Specifically, an inverse shape replication method of making a ceramiccomposite article is disclosed in Commonly Owned U.S. patent applicationSer. No. 823,542, filed Jan. 27, 1986 in the names of Marc S. Newkirk etal, and entitled "Inverse Shape Replication Method of Making CeramicComposite Articles and Articles Obtained Thereby", now allowed in theUnited States (a foreign counterpart to this Application was publishedin the EPO on Sept. 2, 1987, as application No. 0234704). In accordancewith a method disclosed in this U.S. Patent Application, a shaped parentmetal is embedded in a bed of comformable filler and the shaped parentmetal is induced to form an oxidation reaction product which grows intothe bed of comformable filler, thereby resulting in a ceramic compositebody having a shaped cavity therein which substantially corresponds tothe shape of the original shaped parent metal.

The subject matter of this Commonly Owned Ceramic Matrix PatentApplication is expressly incorporated herein by reference.

SUMMARY OF THE INVENTION

A metal matrix composite body is produced by spontaneously infiltratinga permeable mass of filler material with a molten matrix metal.Specifically, in a preferred embodiment, the filler material comprises acomformable material which at least partically surrounds, initially, ashaped ingot of matrix metal. At some point during the processing, thefiller material may become self-supporting. Specifically, the permeablemass of filler material may become self-supporting by being exposed to,for example, elevated temperatures, and/or a bonding agent, and/or areactant, etc. Moreover, it is preferable that the permeable fillermaterial has sufficient comformability over a particular heating rangeso that it can accommodate any differential thermal expansion betweenitself and the shaped matrix metal plus any melting-point volume changeof the shaped matrix metal.

Moreover, in a preferred embodiment, at least in a support zone thereofwhich surrounds the shaped matrix metal, the filler material may beintrinsically self-bonding, preferably, at a temperature which is abovethe melting point of the shaped matrix metal, but below and preferrably,somewhat close to the temperature at which the matrix metal is mademolten.

Moreover, in a further preferred embodiment, the filler material becomesself-supporting due to a reaction with a component (e.g., aninfiltrating atmosphere) which, at least at some point during thespontaneous process, is exposed to the filler material.

An infiltration enhancer and/or an infiltration enhancer precursorand/or an infiltrating atmosphere are also in communication with thefiller material, at least at some point during the process, whichpermits the shaped matrix metal, when made molten, to spontaneouslyinfiltrate the filler material.

Once a desired amount of spontaneous infiltration of molten matrix metalinto the filler material has been achieved, a cavity, which at leastpartially corresponds to the shape of the shaped ingot of matrix metal,is formed in the spontaneous infiltrated filler material (i.e., themetal matrix composite body which is formed contains a cavity therein).

In one preferred embodiment, the filler material may include aninfiltration enhancer precursor therein. The filler material canthereafter be contacted with an infiltrating atmosphere to form theinfiltration enhancer at least in a portion of the filler material. Suchan infiltration enhancer can be formed prior to or substantiallycontiguous with contacting of the molten matrix metal with the fillermaterial. Moreover, an infiltrating atmosphere may be provided duringsubstantially all of the spontaneous infiltration process and thus be incommunication with a filler material or alternatively, may communicatewith the filler material and/or matrix metal for only a portion of thespontaneous infiltration process. Ultimately, it is desirable that atleast during the spontaneous infiltration, the least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of a filler material.

Moreover, in a further preferred embodiment invention, rather thansupplying an infiltration enhancer precursor to the filler material, aninfiltration enhancer may be supplied directly to at least one of thefiller material and/or matrix metal, and/or infiltrating atmosphere.Again, ultimately, at least during the spontaneous infiltration, theinfiltration enhancer should be located in at least a portion of thefiller material.

It is noted that this application discusses primarily aluminum matrixmetals which, at some point during the formation of the metal matrixcomposite body, are contacted with magnesium, which functions as theinfiltration enhancer precursor, in the presence of nitrogen, whichfunctions as the infiltrating atmosphere. Thus, the matrixmetal/infiltration enhancer precursor/infiltrating atmosphere system ofaluminum/magnesium/nitrogen exhibits spontaneous infiltration. However,other matrix metal/infiltration enhancer precursor/infiltratingatmosphere systems may also behave in a manner similar to the systemaluminum/magnesium/nitrogen. For example, similar spontaneousinfiltration behavior has been observed in thealuminum/strontium/nitrogen system; the aluminum/zinc/oxygen system; andthe aluminum/calcium/nitrogen system. Accordingly, even though thealuminum/magnesium/nitrogen system is discussed primarily herein, itshould be understood that other matrix metal/infiltration enhancerprecursor/infiltrating atmosphere systems may behave in a similar mannerand are intended to be encompassed by the invention.

When the matrix metal comprises an aluminum alloy, the aluminum alloy iscontacted with a filler material (e.g., alumina or silicon carbideparticles), said filler material having admixed therewith, and/or atsome point during the process being exposed to, magnesium. Moreover, ina preferred embodiment, the aluminum alloy and/or preform or fillermaterial are contained in a nitrogen atmosphere for at least a portionof the process. The preform will be spontaneously infiltrated and theextent or rate of spontaneous infiltration and formation of metal matrixwill vary with a given set of process conditions including, for example,the concentration of magnesium provided to the system (e.g., in thealuminum alloy and/or in the filler material and/or in the infiltratingatmosphere), the size and/or composition of the particles in the fillermaterial, the concentration of nitrogen in the infiltrating atmosphere,the time permitted for infiltration, and/or the temperature at whichinfiltration occurs. Spontaneous infiltration typically occurs to anextent sufficient to embed substantially completely the filler material.

DEFINITIONS

"Aluminum", as used herein, means and includes essentially pure metal(e.g., a relatively pure, commercially available unalloyed aluminum) orother grades of metal and metal alloys such as the commerciallyavailable metals having impurities and/or alloying constituents such asiron, silicon, copper, magnesium, manganese, chromium, zinc, etc.,therein. An aluminum alloy for purposes of this definition is an alloyor intermetallic compound in which aluminum is the major constituent.

"Balance Non-Oxidizing Gas", as used herein, means that any gas presentin addition to the primary gas comprising the infiltrating atmosphere,is either an inert gas or a reducing gas which is substantiallynon-reactive with the matrix metal under the process conditions. Anyoxidizing gas which may be present as an impurity in the gas(es) usedshould be insufficient to oxidize the matrix metal to any substantialextent under the process conditions.

"Barrier" or "barrier means," as used herein, means any suitable meanswhich interferes, inhibits, prevents or terminates the migration,movement, or the like, of molten matrix metal beyond a surface boundaryof a permeable mass of filler material or preform, where such surfaceboundary is defined by said barrier means. Suitable barrier means may beany such material, compound, element, composition, or the like, which,under the process conditions, maintains some integrity and is notsubstantially volatile (i.e., the barrier material does not volatilizeto such an extent that it is rendered non-functional as a barrier).

Further, suitable "barrier means" includes materials which aresubstantially non-wettable by the migrating molten matrix metal underthe process conditions employed. A barrier of this type appears toexhibit substantially little or no affinity for the molten matrix metal,and movement beyond the defined surface boundary of the mass of fillermaterial or preform is prevented or inhibited by the barrier means. Thebarrier reduces any final machining or grinding that may be required anddefines at least a portion of the surface of the resulting metal matrixcomposite product. The barrier may in certain cases be permeable orporous, or rendered permeable by, for example, drilling holes orpuncturing the barrier, to permit gas to contact the molten matrixmetal.

"Carcass" or "Carcass of Matrix Metal" as used herein, refers to any ofthe original body of matrix metal remaining which has not been consumedduring formation of the metal matrix composite body, and typically, ifallowed to cool, remains in at least partial contact with the metalmatrix composite body which has been formed. It should be understoodthat the carcass may also include a second or foreign metal therein.

"Cavity", as used herein, means any unfilled space within a mass or body(e.g., a metal matrix composite) and is not limited to any specificconfiguration of space and includes both enclosed and open spaces.Specifically, a cavity can include those spaces which are entirelyclosed off from communication with an exterior portion of the mass orbody containing the cavity, such as a cavity defining the interior of ahollow body. Moreover, a cavity can include those spaces which are opento an external surface of a mass or body by, for example, a passagewayor opening.

"Filler" as used herein, is intended to include either singleconstituents or mixtures of constituents which are substantiallynon-reactive with and/or of limited solubility in the matrix metal andmay be single or multi-phase. Fillers may be provided in a wide varietyof forms, such as powders, flakes, platelets, microspheres, whiskers,bubbles, etc., and may be either dense or porous. "Filler" may alsoinclude ceramic fillers, such as alumina or silicon carbide as fibers,chopped fibers, particulates, whiskers, bubbles, spheres, fiber mats, orthe like, and coated fillers such as carbon fibers coated with aluminaor silicon carbide to protect the carbon from attack, for example, by amolten aluminum matrix metal. Fillers may also include metals in anydesired configuration.

"Infiltrating Atmosphere", as used herein, means that atmosphere whichis present which interacts with the matrix metal and/or preform (orfiller material) and/or infiltration enhancer precursor and/orinfiltration enhancer and permits or enhances spontaneous infiltrationof the matrix metal to occur.

"Infiltration Enhancer", as used herein, means a material which promotesor assists in the spontaneous infiltration of a matrix metal into afiller material or preform. An infiltration enhancer may be formed from,for example, a reaction of an infiltration enhancer precursor with aninfiltrating atmosphere to form (1) a gaseous species and/or (2) areaction product of the infiltration enhancer precursor and theinfiltrating atmosphere and/or (3) a reaction product of theinfiltration enhancer precursor and the filler material or preform.Moreover, the infiltration enhancer may be supplied directly to at leastone of the preform, and/or matrix metal, and/or infiltrating atmosphereand function in a substantially similar manner to an infiltrationenhancer which has formed as a reaction between an infiltration enhancerprecursor and another species. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform to achievespontaneous infiltration.

"Infiltration Enhancer Precursor" or "Precursor to the InfiltrationEnhancer", as used herein, means a material which when used incombination with the matrix metal, preform and/or infiltratingatmosphere forms an infiltration enhancer which induces or assists thematrix metal to spontaneously infiltrate the filler material or preform.Without wishing to be bound by any particular theory or explanation, itappears as though it may be necessary for the precursor to theinfiltration enhancer to be capable of being positioned, located ortransportable to a location which permits the infiltration enhancerprecursor to interact with the infiltrating atmosphere and/or thepreform or filler material and/or metal. For example, in some matrixmetal/infiltration enhancer precursor/infiltrating atmosphere systems,it is desirable for the infiltration enhancer precursor to volatilizeat, near, or in some cases, even somewhat above the temperature at whichthe matrix metal becomes molten. Such volatilization may lead to: (1) areaction of the infiltration enhancer precursor with the infiltratingatmosphere to form a gaseous species which enhances wetting of thefiller material or preform by the matrix metal; and/or (2) a reaction ofthe infiltration enhancer precursor with the infiltrating atmosphere toform a solid, liquid or gaseous infiltration enhancer in at least aportion of the filler material or preform which enhances wetting; and/or(3) a reaction of the infiltration enhancer precursor within the fillermaterial or preform which forms a solid, liquid or gaseous infiltrationenhancer in at least a portion of the filler material or preform whichenhances wetting.

"Matrix Metal" or "Matrix Metal Alloy", as used herein, means that metalwhich is utilized to form a metal matrix composite (e.g., beforeinfiltration) and/or that metal which is intermingled with a fillermaterial to form a metal matrix composite body (e.g., afterinfiltration). When a specified metal is mentioned as the matrix metal,it should be understood that such matrix metal includes that metal as anessentially pure metal, a commercially available metal having impuritiesand/or alloying constituents therein, an intermetallic compound or analloy in which that metal is the major or predominant constituent.

"Matrix Metal/Infiltration Enhancer Precursor/Infiltrating AtmosphereSystem" or "Spontaneous System,", as used herein, refers to thatcombination of materials which exhibits spontaneous infiltration into apreform or filler material. It should be understood that whenever a "/"appears between an exemplary matrix metal, infiltration enhancerprecursor and infiltrating atmosphere, the "/" is used to designate asystem or combination of materials which, when combined in anappropriate manner, exhibits spontaneous infiltration into a preform orfiller material.

"Metal Matrix Composite" or "MMC", as used herein, means a materialcomprising a two- or three-dimensionally interconnected alloy or matrixmetal which has embedded a preform or filler material. The matrix metalmay include various alloying elements to provide specifically desiredmechanical and physical properties in the resulting composite.

A Metal "Different" from the Matrix Metal means a metal which does notcontain, as a primary constituent, the same metal as the matrix metal(e.g., if the primary constituent of the matrix metal is aluminum, the"different" metal could have a primary constituent of, for example,nickel).

"Nonreactive Vessel for Housing Matrix Metal" means any vessel which canhouse or contain a filler material and/or molten matrix metal under theprocess conditions and not react with the matrix and/or the infiltratingatmosphere and/or infiltration enhancer precursor and/or filler materialor preform in a manner which would be significantly detrimental to thespontaneous infiltration mechanism.

"Preform" or "Permeable Preform", as used herein, means a porous mass offiller or filler material which is manufactured with at least onesurface boundary which essentially defines a boundary for infiltratingmatrix metal, such mass retaining sufficient shape integrity and greenstrength to provide dimensional fidelity prior to being infiltrated bythe matrix metal. The mass should be sufficiently porous to accommodatespontaneous infiltration of the matrix metal thereinto. A preformtypically comprises a bonded array or arrangement of filler, eitherhomogeneous or heterogeneous, and may be comprised of any suitablematerial (e.g., ceramic and/or metal particulates, powders, fibers,whiskers, etc., and any combination thereof). A preform may exist eithersingularly or as an assemblage.

"Reservoir", as used herein, means a seaparate body of matrix metalpositioned relative to a mass of filler or a preform so that, when themetal is molten, it may flow to replenish, or in some cases to initiallyprovide and subsequently replenish, that portion, segment or source ofmatrix metal which is in contact with the filler or preform. Thereservoir may also be used to provide a metal which is different fromthe matrix metal.

"Shaped Matrix Metal" or "Ingot of Shaped Matrix Metal", as used herein,means a matrix metal which has been shaped into a predetermined patternwhich, under the process conditions of the present invention, willspontaneously infiltrate a surrounding filler material, thereby forminga metal matrix composite which at least partially inversely replicatesthe configuration of the shaped matrix metal.

"Spontaneous Infiltration", as used herein, means the infiltration of amatrix metal into a permeable mass of filler or preform that occurswithout the requirement of application of pressure of vacuum (whetherexternally applied or internally created).

BRIEF DESCRIPTION OF THE FIGURES

The following figures are provided to assist in understanding theinvention, but are not intended to limit the scope of the invention.Similar reference numerals have been used wherever possible in each ofthe Figures to denote like components, wherein:

FIG. 1 is a schematic cross section of an assemblage of materialsutilized in accordance with Example 1.

FIG. 2 is a schematic cross section of an assemblage of materialsutilized in accordance with Example 2.

FIGS. 3A and 3B are photographs of the metal matrix composite producedin accordance with Example 1.

FIGS. 4A and 4B are photographs of the metal matrix composite producedin accordance with Example 2.

DETAILED DESCRIPTION OF THE INVENTION AND PREFERRED EMBODIMENTS

The present invention relates to forming a metal matrix composite havingtherein a cavity which has been formed by a shape replication process ofan ingot of matrix metal. Particularly, an ingot of matrix metal may beshaped into a predetermined shape and surrounded, at least partially,by, a filler material.

The filler can completely, or only partially, surround the shaped matrixmetal ingot, or a portion of the shaped ingot can extend outwardlybeyond the filler. However, such outwardly extending portion of theshaped ingot will not be replicated. Further, a barrier means, discussedin greater detail later herein, can be used to provide a non-replicatingsurface portion when said barrier means contacts at least a portion of asurface of said shaped matrix metal ingot. Accordingly, the presentinvention permits the formation of a metal matrix composite whichinversely replicates a shaped ingot of matrix metal to any desiredextent.

To achieve spontaneous infiltration, an infiltration enhancer and/or aninfiltration enhancer precursor and/or infiltrating atmosphere are incommunication with the filler material, at least at some point duringthe process, which permits the matrix metal, when made molten, tospontaneously infiltrate the filler material. After such spontaneousinfiltration has been achieved, a cavity is formed in a metal matrixcomposite body, said cavity being at least partially complementary tothe original shape of the shaped matrix metal ingot.

In order to effect spontaneous infiltration of the matrix metal into thefiller material or preform, an infiltration enhancer should be providedto the spontaneous system. An infiltration enhancer could be formed froman infiltration enhancer precursor which could be provided (1) in thematrix metal; and/or (2) in the filler material and/or (3) from theinfiltrating atmosphere and/or (4) from an external source into thespontaneous system. Moreover, rather than supplying an infiltrationenhancer precursor, an infiltration enhancer may be supplied directly toat least one of the filler material or preform, and/or matrix metal,and/or infiltrating atmosphere. Ultimately, at least during thespontaneous infiltration, the infiltration enhancer should be located inat least a portion of the filler material or preform.

In a preferred embodiment it is possible that the infiltration enhancerprecursor can be at least partially reacted with the infiltratingatmosphere such that the infiltration enhancer can be formed in at leasta portion of the filler material prior to or substantiallysimultaneously with contacting the filler material with molten matrixmetal (e.g., if magnesium was the infiltration enhancer precursor andnitrogen was the infiltrating atmosphere, the infiltration enhancercould be magnesium nitride which would be located in at least a portionof the filler material).

An example of a matrix metal/infiltration enhancerprecursor/infiltrating atmosphere system is thealuminum/magnesium/nitrogen system. Specifically, a shaped ingot ofaluminum matrix metal can be embedded within a filler material which canbe contained within a suitable refractory vessel which, under theprocess conditions, does not react with the aluminum matrix metal and/orthe filler material when the aluminum is made molten. A filler materialcontaining or being exposed to magnesium, and being exposed to, at leastat some point during the processing, a nitrogen atmosphere, can becontacted with the molten aluminum matrix metal. The matrix metal willthen spontaneously infiltrate the filler material.

Under the conditions employed in the method of the present invention, inthe case of an aluminum/magnesium/nitrogen spontaneous infiltrationsystem, the filler material should be sufficiently permeable to permitthe nitrogen-containing gas to penetrate or permeate the filler materialat some point during the process and/or contact the molten matrix metal.Moreover, the permeable filler material can accommodate infiltration ofthe molten matrix metal, thereby causing the nitrogen-permeated fillermaterial to be infiltrated spontaneously with molten matrix metal toform a metal matrix composite body and/or cause the nitrogen to reactwith an infiltration enhancer precursor to form infiltration enhancer inthe filler material and thereby result in spontaneous infiltration. Theextent or rate of spontaneous infiltration and formation of the metalmatrix composite will vary with a given set of process conditions,including the magnesium content of the aluminum alloy, magnesium contentof the filler material, amount of magnesium nitride in the fillermaterial, the presence of additional alloying elements (e.g., silicon,iron, copper, manganese, chromium, zinc, and the like), average size ofthe filler material (e.g., particle diameter), surface condition andtype of filler material, nitrogen concentration of the infiltratingatmosphere, time permitted for infiltration and temperature at whichinfiltration occurs. For example, for infiltration of the moltenaluminum matrix metal to occur spontaneously, the aluminum can bealloyed with at least about 1% by weight, and preferably at least about3% by weight, magnesium (which functions as the infiltration enhancerprecursor), based on alloy weight. Auxiliary alloying elements, asdiscussed above, may also be included in the matrix metal to tailorspecific properties thereof. Additionally, the auxiliary alloyingelements may affect the minimum amount of magnesium required in thematrix aluminum metal to result in spontaneous infiltration of thefiller material. Loss of magnesium from the spontaneous system due to,for example, volatilization should not occur to such an extent that nomagnesium was present to form infiltration enhancer. Thus, it isdesirable to utilize a sufficient amount of initial alloying elements toassure that spontaneous infiltration will not be adversely affected byvolatilization. Still further, the presence of magnesium in both of thefiller material and matrix metal or the filler material alone may resultin a reduction in the required amount of magnesium to achievespontaneous infiltration (discussed in greater detail later herein).

The volume percent of nitrogen in the nitrogen atmosphere also affectsformation rates of the metal matrix composite body. Specifically, ifless than about 10 volume percent of nitrogen is present in theinfiltrating atmosphere, very slow or little spontaneous infiltrationwill occur. It has been discovered that it is preferable for at leastabout 50 volume percent of nitrogen to be present in the atmosphere,thereby resulting in, for example, shorter infiltration times due to amuch more rapid rate of infiltration. The infiltrating atmosphere (e.g.,a nitrogen-containing gas) can be supplied directly to the fillermaterial or preform and/or matrix metal, or it may be produced or resultfrom a decomposition of a material.

The minimum magnesium content required for molten aluminum matrix metalto infiltrate a filler material or preform depends on one or morevariables such as the processing temperature, time, the presence ofauxiliary alloying elements such as silicon or zinc, the nature of thefiller material, the location of the magnesium in one or more componentsof the spontaneous system, the nitrogen content of the atmosphere, andthe rate at which the nitrogen atmosphere flows. Lower temperatures orshorter heating times can be used to obtain complete infiltration as themagnesium content of the alloy and/or filler material is increased.Also, for a given magnesium content, the addition of certain auxiliaryalloying elements such as zinc permits the use of lower temperatures.For example, a magnesium content of the matrix metal at the lower end ofthe operable range, e.g., from about 1 to 3 weight percent, may be usedin conjunction with at least one of the following: an above-minimumprocessing temperature, a high nitrogen concentration, or one or moreauxiliary alloying elements. When no magnesium is added to the fillermaterial, alloys containing from about 3 to 5 weight percent magnesiumare preferred on the basis of their general utility over a wide varietyof process conditions, with at least about 5 percent being preferredwhen lower temperatures and shorter times are employed. Magnesiumcontents in excess of about 10 percent by weight of the aluminum alloymay be employed to moderate the temperature conditions required forinfiltration. The magnesium content may be reduced when used inconjunction with an auxiliary alloying element, but these elements servean auxiliary function only and are used together with at least theabove-specified minimum amount of magnesium. For example, there wassubstantially no infiltration of nominally pure aluminum alloyed onlywith 10 percent silicon at 1000° C. into a bedding of 500 mesh, 39Crystolon (99 percent pure silicon carbide from Norton Co.). However, inthe presence of magnesium, silicon can promote the infiltration process.As a further example, the amount of magnesium varies if it is suppliedexclusively to the filler material. It has been discovered thatspontaneous infiltration will occur with a lesser weight percent ofmagnesium supplied to the spontaneous system when at least some of thetotal amount of magnesium supplied is placed in the filler material. Itmay be desirable for a lesser amount of magnesium to be provided inorder to prevent the formation of undesirable intermetallics in themetal matrix composite body. In the case of a silicon carbide preform,it has been discovered that when the preform is contacted with analuminum matrix metal, the preform containing at least about 1% byweight magnesium and being in the presence of a substantially purenitrogen atmosphere, the matrix metal spontaneously infiltrates thepreform. In the case of an alumina preform, the amount of magnesiumrequired to achieve acceptable spontaneous infiltration is slightlyhigher. Specifically, it has been found that when an alumina preform iscontacted with a similar aluminum matrix metal at about the sametemperature as the aluminum that infiltrated into the silicon carbidepreform, and, in the presence of the same nitrogen atmosphere, at leastabout 3% by weight magnesium may be required to achieve similarspontaneous infiltration to that achieved in the silicon carbide preformdiscussed immediately above.

It is also noted that it is possible to supply to the spontaneous systeminfiltration enhancer precursor and/or infiltration enhancer on asurface of the alloy and/or on a surface of the filler material and/orwithin the filler material prior to infiltrating the matrix metal intothe filler material (i.e., it may not be necessary for the suppliedinfiltration enhancer or infiltration enhancer precursor to be alloyedwith the matrix metal, but rather, simply supplied to the spontaneoussystem). If the magnesium was applied to a surface of the matrix metalit may be preferred that said surface should be the surface which isclosest to, or preferably in contact with, the permeable mass of fillermaterial or vice versa; or such magnesium could be mixed into at least aportion of the filler material. Still further, it is possible that somecombination of surface application, alloying and placement of magnesiuminto at least a portion of the filler material could be used. Suchcombination of applying infiltration enhancer(s) and/or infiltrationenhancer precursor(s) could result in a decrease in the total weightpercent of magnesium needed to promote infiltration of the matrixaluminum metal into the filler material, as well as achieving lowertemperatures at which infiltration can occur. Moreover, the amount ofundesirable intermetallics formed due to the presence of magnesium couldalso be minimized.

The use of one or more auxiliary alloying elements and the concentrationof nitrogen in the surrounding gas also affects the extent of nitridingof the matrix metal at a given temperature. For example, auxiliaryalloying elements such as zinc or iron included in the shaped alloy, orplaced on a surface of the shaped alloy, may be used to reduce theinfiltration temperature and thereby decrease the amount of nitrideformation, whereas increasing the concentration of nitrogen in the gasmay be used to promote nitride formation.

The concentration of magnesium in the alloy, and/or placed onto asurface of the alloy, and/or combined in the filler material, also tendsto affect the extent of infiltration at a given temperature.Consequently, in some cases where little or no magnesium is contacteddirectly with the filler material, it may be preferred that at leastabout three weight percent magnesium be included in the alloy. Alloycontents of less than this amount, such as one weight percent magnesium,may require higher process temperatures or an auxiliary alloying elementfor infiltration. The temperature required to effect the spontaneousinfiltration process of this invention may be lower: (1) when themagnesium content of the alloy alone is increased, e.g. to at leastabout 5 weight percent; and/or (2) when alloying constituents are mixedwith the permeable mass of filler material; and/or (3) when anotherelement such as zinc or iron is present in the aluminum alloy. Thetemperature also may vary with different filler materials. In general,spontaneous and progressive infiltration will occur at a processtemperature of at least about 675° C., and preferably a processtemperature of at least about 750° C.-800° C. Temperatures generally inexcess of 1200° C. do not appear to benefit the process, and aparticularly useful temperature range has been found to be from about675° C. to about 1200° C. However, as a general rule, the spontaneousinfiltration temperature is a temperature which is above the meltingpoint of the matrix metal but below the volatilization temperature ofthe matrix metal. Moreover, the spontaneous infiltration temperatureshould be below the melting point of the filler material. Still further,as temperature is increased, the tendency to form a reaction productbetween the matrix metal and infiltrating atmosphere increases (e.g., inthe case of aluminum matrix metal and a nitrogen infiltratingatmosphere, aluminum nitride may be formed). Such reaction product maybe desirable or undesirable, dependent upon the intended application ofthe metal matrix composite body. Additionally, electric heating istypically used to achieve the infiltrating temperatures. However, anyheating means which can cause the matrix metal to become molten and doesnot adversely affect spontaneous infiltration, is acceptable for usewith the invention.

In the present method, for example, a permeable filler material comesinto contact with molten aluminum in the presence of, at least some timeduring the process, a nitrogen-containing gas. The nitrogen-containinggas may be supplied by maintaining a continuous flow of gas into contactwith at least one of the filler material and/or molten aluminum matrixmetal. Although the flow rate of the nitrogen-containing gas is notcritical, it is preferred that the flow rate be sufficient to compensatefor any nitrogen lost from the atmosphere due to nitride formation inthe alloy matrix, and also to prevent or inhibit the incursion of airwhich can have an oxidizing effect on the molten metal.

The method of forming a metal matrix composite is applicable to a widevariety of filler materials, and the choice of filler materials willdepend on such factors as the matrix alloy, the process conditions, thereactivity of the molten matrix alloy with the filler material, theability of the filler material to conform to the shape of the shapedingot of matrix metal and the properties sought for the final compositeproduct. For example, when aluminum is the matrix metal, suitable fillermaterials include (a) oxides, e.g. alumina; (b) carbides, e.g. siliconcarbide; (c) borides, e.g. aluminum dodecaboride, and (d) nitrides, e.g.aluminum nitride. If there is a tendency for the filler material toreact with the molten aluminum matrix metal, this might be accommodatedby minimizing the infiltration time and temperature or by providing anon-reactive coating on the filler. The filler material may comprise asubstrate, such as carbon or other non-ceramic material, bearing acoating to protect the substrate from attack or degradation. Suitablecoatings can be ceramic such as oxides, carbides, borides and nitrides.Ceramics which are preferred for use in the present method includealumina and silicon carbide in the form of particles, platelets,whiskers and fibers. The fibers can be discontinuous (in chopped form)or in the form of continuous filament, such as multifilament tows.Further, the filler material may be homogeneous or heterogeneous.

It also has been discovered that certain filler materials exhibitenhanced infiltration relative to filler materials by having a similarchemical composition. For example, crushed alumina bodies made by themethod disclosed in U.S. Pat. No. 4,713,360, entitled "Novel CeramicMaterials and Methods of Making Same", which issued on Dec. 15, 1987, inthe names of Marc S. Newkirk et al., exhibit desirable infiltrationproperties relative to commercially available alumina products.Moreover, crushed alumina bodies made by the method disclosed inCopending and Commonly Owned Application Ser. No. 819,397 entitled"Composite Ceramic Articles and Methods of Making Same", in the names ofMarc S. Newkirk et al, also exhibit desirable infiltration propertiesrelative to commerically available alumina products. The subject matterof each of the issued Patent and Copending Patent Application is hereinexpressly incorporated by reference. Thus, it has been discovered thatcomplete infiltration of a permeable mass of ceramic material can occurat lower infiltration temperatures and/or lower infiltration times byutilizing a crushed or comminuted body produced by the method of theaforementioned U.S. Patent and Patent Application.

The size and shape of the filler material can be any that may berequired to achieve the properties desired in the composite and whichcan conform to the shaped ingot of matrix metal. Thus, the fillermaterial may be in the form of particles, whiskers, platelets or fiberssince infiltration is not restricted by the shape of the fillermaterial. Other shapes such as spheres, tubules, pellets, refractoryfiber cloth, and the like may be employed. In addition, the size of thematerial does not limit infiltration, although a higher temperature orlonger time period may be needed for complete infiltration of a mass ofsmaller particles than for larger particles. Further, the fillermaterial to be infiltrated should be permeable to the molten matrixmetal and to the infiltrating atmosphere.

The method of forming metal matrix composites according to the presentinvention is advantageously not dependent upon the use of pressure toforce or squeeze molten matrix metal into a mass of filler material. Theinvention permits the production of substantially uniform metal matrixcomposites having a high volume fraction of filler material and lowporosity. Higher volume fractions of filler material on the order of atleast about 50% may be achieved by using a lower porosity initial massof filler material. Higher volume fractions also may be achieved if themass of filler is compacted or otherwise densified provided that themass is not converted into either a compact with close cell porosity orinto a fully dense structure that would prevent infiltration by themolten alloy.

It has been observed that for aluminum infiltration and matrix formationaround a ceramic filler, wetting of the ceramic filler by the aluminummatrix metal may be an important part of the infiltration mechanism.Moreover, at low processing temperatures, a negligible or minimal amountof metal nitriding occurs resulting in a minimal discontinuous phase ofaluminum nitride dispersed in the metal matrix. However, as the upperend of the temperature range is approached, nitridation of the metal ismore likely to occur. Thus, the amount of the nitride phase in the metalmatrix can be controlled by varying the processing temperature at whichinfiltration occurs. The specific process temperature at which nitrideformation becomes more pronounced also varies with such factors as thematrix aluminum alloy used and its quantity relative to the volume offiller material, the filler material to be infiltrated, and the nitrogenconcentration of the infiltrating atmosphere. For example, the extent ofaluminum nitride formation at a given process temperature is believed toincrease as the ability of the alloy to wet the filler decreases and asthe nitrogen concentration of the atmosphere increases.

It is therefore possible to tailor the constituency of the metal matrixduring formation of the composite to impart certain characteristics tothe resulting product. For a given system, the process conditions can beselected to control the nitride formation. A composite productcontaining an aluminum nitride phase will exhibit certain propertieswhich can be favorable to, or improve the performance of, the product.Further, the temperature range for spontaneous infiltration with analuminum alloy may vary with the filler material used. In the case ofalumina as a filler material, the temperature for infiltration shouldpreferably not exceed about 1000° C. if it is desired that the ductilityof the matrix not be reduced by the significant formation of nitride.However, temperatures exceeding 1000° C. may be employed if it isdesired to produce a composite with a less ductile and stiffer matrix.To infiltrate silicon carbide, higher temperatures of about 1200° C. maybe employed since the aluminum alloy nitrides to a lesser extent,relative to the use of alumina as filler, when silicon carbide isemployed as a filler material.

Moreover, it is possible to use a reservoir of matrix metal to assurecomplete infiltration of the filler material and/or to supply a secondmetal which has a different composition from the first source of matrixmetal. Specifically, in some cases it may be desirable to utilize amatrix metal in the reservoir which differs in composition from thefirst source of matrix metal. For example, if an aluminum alloy is usedas the first source of matrix metal, then virtually any other metal ormetal alloy which was molten at the processing temperature could be usedas the reservoir metal. Molten metals frequently are very miscible witheach other which would result in the reservoir metal mixing with thefirst source of matrix metal so long as an adequate amount of time isgiven for the mixing to occur. Thus, by using a reservoir metal which isdifferent in composition than the first source of matrix metal, it ispossible to tailor the properties of the metal matrix to meet variousoperating requirements and thus tailor the properties of the metalmatrix composite.

A barrier means may also be utilized in combination with the presentinvention. Specifically, the barrier means for use with this inventionmay be any suitable means which interferes, inhibits, prevents orterminates the migration, movement, or the like, of molten matrix alloy(e.g., an aluminum alloy) beyond the defined surface boundary of thefiller material. Suitable barrier means may be any material, compound,element, composition, or the like, which, under the process conditionsof this invention, maintains some integrity, is not volatile andpreferably is permeable to the gas used with the process as well asbeing capable of locally inhibiting, stopping, interfering with,preventing, or the like, continued infiltration or any other kind ofmovement beyond the defined surface boundary of the filler material.

Suitable barrier means includes materials which are substantiallynon-wettable by the migrating molten matrix alloy under the processconditions employed. A barrier of this type appears to exhibit little orno affinity for the molten matrix alloy, and movement beyond the definedsurface boundary of the filler material is prevented or inhibited by thebarrier means. The barrier reduces any final machining or grinding thatmay be required of the metal matrix ceramic composite product. As statedabove, the barrier preferably should be permeable or porous, or renderedpermeable by puncturing, to permit the gas to contact the molten matrixalloy.

Suitable barriers particularly useful for aluminum matrix alloys arethose containing carbon, especially the crystalline allotropic form ofcarbon known as graphite. Graphite is essentially non-wettable by themolten aluminum alloy under the described process conditions. Aparticularly preferred graphite is a graphite tape product that is soldunder the trademark Grafoil®, registered to Union Carbide. This graphitetape exhibits sealing characteristics that prevent the migration ofmolten aluminum alloy beyond the defined surface boundary of the fillermaterial. This graphite tape is also resistant to heat and is chemicallyinert. Grafoil® graphite material is flexible, compatible, conformableand resilient. It can be made into a variety of shapes to fit anybarrier application. However, graphite barrier means may be employed asa slurry or paste or even as a paint film around and on the boundary ofthe filler material. Grafoil® is particularly preferred because it is inthe form of a flexible graphite sheet. In use, this paper-like graphiteis simply formed around the filler material.

Other preferred barrier(s) for infiltrating aluminum metal matrix alloysin a nitrogen environment are the transition metal borides (e.g.,titanium diboride (TiB₂)) which are generally non-wettable by the moltenaluminum metal alloy under certain of the process conditions employedusing this material. With a barrier of this type, the processtemperature should not exceed about 875° C., for otherwise the barriermaterial becomes less efficacious and, in fact, with increasedtemperature infiltration into the barrier will occur. The transitionmetal borides are typically in a particulate form (1-30 microns). Themetal formation may be applied as a slurry or paste to the boundaries ofthe permeable mass of ceramic filler material which preferably ispreshaped as a preform.

Other useful barriers for aluminum metal matrix alloys in nitrogeninclude low-volatile organic compounds applied as a film or layer ontothe external surface of the filler material. Upon firing in nitrogen,especially at the process conditions of this invention, the organiccompound decomposes leaving a carbon soot film. The organic compound maybe applied by conventional means such as painting, spraying, dipping,etc.

Moreover, finely ground particulate materials can function as a barrierso long as infiltration of the particulate material would occur at arate which is slower than the rate of infiltration of the fillermaterial.

Thus, the barrier means may be applied by any suitable means, such as bycovering the defined surface boundary with a layer of the barrier means.Such a layer of barrier means may be applied by painting, dipping, silkscreening, evaporating, or otherwise applying the barrier means inliquid, slurry, or paste form, or by sputtering a vaporizable barriermeans, or by simply depositing a layer of a solid particulate barriermeans, or by applying a solid thin sheet or film of barrier means ontothe defined surface boundary. With the barrier means in place,spontaneous infiltration substantially terminates when the infiltratingmatrix metal reaches the defined surface boundary and contacts thebarrier means.

Various demonstrations of the present invention are included in theExamples immediately following. However, these Examples should beconsidered as being illustrative and should not be construed as limitingthe scope of the invention as defined in the appended claims.

EXAMPLE 1

FIG. 1 shows an assembly, in cross section, which can be used to form ashaped cavity in a metal matrix composite. Particularly, a fillermaterial (3) comprising 220 grit silicon carbide supplied by Norton Co.,and sold under the trade name of 39 Crystolon, was placed into arefractory vessel (1) comprising a high purity alumina boat. The aluminarefractory boat was obtained from Bolt Technical Ceramics, and had apurity of 99.7%. Two shaped aluminum alloy bars (2a) and (2b), eachcomprising about 15 percent by weight silicon and about 15 percent byweight magnesium, and a remainder being aluminum, and each measuringabout 41/2 inches by 2 inches by 1/2 inch, were stacked on top of eachother and were embedded into the 220 grit silicon carbide such that asurface of the bar (2a) was substantially flush with a surface of thefiller material (3). The alumina boat (1) containing the filler material(3) and ingots (2a) and (2 b) was placed into a controlled atmosphereelectric resistance furnace. Particularly, the furnace comprised amuffle tube which was externally heated by a resistance coil andfurther, was sealed from the external atmosphere. An infiltratingatmosphere comprising about 96% by volume nitrogen and about 4% byvolume hydrogen (i.e., forming gas) was supplied to the inside of themuffle tube. The forming gas flowed into the furnace at a rate of about350 cc/min. The muffle furnace was brought up to a temperature of about900°-930° C. over a period of about 10 hours. This temperature wasmaintained for about 12 hours and the muffle furnace was cooled to aboutroom temperature over a five hour period of time.

The boat (1) was removed from the furnace and the contents wereinspected. As shown in FIG. 3a, which is an overhead view of the formedmetal matrix composite (7), a cavity (6), which substantiallycorresponded in shape to the shaped ingots (2a) and (2b), was formed.Moreover, as shown in FIG. 3B, which is an angled overhead view lookinginto the cavity (6) in the formed metal matrix composite (7), thereplication of the bars (2a) and (2b) was so accurate that saw marks(8), which were present on the bars (2a) and (2b), were inverselyreplicated in the metal matrixc composite body.

EXAMPLE 2

In this Example, a more complicated shape was inversely replicated. FIG.2 discloses the assembly, in cross section, which was utilized to form acomplex cavity in a metal matrix composite. Specifically, a fillermaterial (5) comprising 220 grit aluminum oxide powder, supplied byNorton Co., and sold under the trade name of 38 Alundum, was poured intoa bottom portion of a refractory vessel (1) comprising a high purityalumina boat. The alumina boat was obtained from Bolt Technical Ceramicsand had a purity of 99.7%. Next, a machined aluminum alloy ingot (4),weighing about 158 grams and containing on an outer surface thereof aplurality of protrusions (9), was placed on top the filler material (5).The machined ingot (4) comprised about 5 percent by weight silicon,about 5 percent by weight zinc, about 7 percent by weight Mg, and theremainder being aluminum. Additional filler material (5) was then pouredaround the ingot (4) until the ingot was substantially completelycovered with filler material (5). The boat (1) containing the fillermaterial (5) and the ingot (4) was then placed into the muffle tubefurnace described in Example 1. A vacuum was then applied in the furnaceto purge the atmosphere therefrom and after such purging an infiltratingatmosphere comprising forming gas (i.e., 96% by volume nitrogen and 4%by volume hydrogen) was flowed into the furnace. The forming gas wascontinuously supplied to the muffle tube furnace at a rate of about 500cc/min. The muffle tube was heated at a rate of about 150° C. per hourup to a temperature of about 875° C. This temperature was maintained forabout 15 hours. The muffle tube furnace was then cooled down to aboutroom temperature at a rate of about 200° C. per hour. After cooling, theboat (1) was removed and inspected.

As shown in FIG. 4a, which is a cross-sectional view of the formed metalmatrix composite, a cavity (10) was formed in a metal matrix compositebody (11), said cavity (10) being substantially complementary in shapeto the ingot (4). Particularly, the matrix metal, when made molten,substantially completely infiltrated the filler material (5) such thatgrooves (9a) were formed as complements to the protrusions (9) on theingot (4). Moreover, FIG. 4B shows an end view of the formed metalmatrix composite (11) prior to being cross sectioned. Accordingly, it isobserved that the replication process provided a composite having acavity (10) which substantially inversely replicated the shaped ingot(4). It is noted that the piece of material (20) located in a bottomportion of the cavity (10) corresponds to a portion of filler materialwhich was located directly above the shaped ingot (4).

While the preceding Examples have been described with particularity,various modifications to these Examples may occur to an artisan ofordinary skill, and all such modifications should be considered to bewithin the scope of the claims appended hereto.

What is claimed is:
 1. A method for making a metal matrix composite,comprising:forming a shaped ingot of matrix metal; at least partiallysurrounding said ingot with a substantially non-reactive filler; heatingat least said ingot to render it molten, thereby forming a source ofmolten matrix metal; and spontaneously infiltrating at least a portionof the filler material with molten matrix metal, thereby forming acavity in said infiltrated filler which corresponds in shape, at leastin part, to said shaped ingot of said matrix metal.
 2. The method ofclaim 1, further comprising the step of providing an infiltratingatmosphere in communication with at least one of the filler and thematrix metal for at least a portion of the period of infiltration. 3.The method of claim 2, wherein the infiltrating atmosphere comprises anatmosphere selected from the group consisting of oxygen and nitrogen. 4.The method of claim 2, further comprising the step of supplying at leastone of an infiltration enhancer precursor and an infiltration enhancerto at least one of the matrix metal, the filler and the infiltratingatmosphere.
 5. The method of claim 4, wherein the infiltration enhanceris formed by reacting an infiltration enhancer precursor and at leastone species selected from the group consisting of the infiltratingatmosphere, a reactive material combined with the filler and the matrixmetal.
 6. The method of claim 5, wherein during infiltration, theinfiltration enhancer precursor volatilizes.
 7. The method of claim 6,wherein the volatilized infiltration enhancer precursor reacts to form areaction product in at least a portion of the filler.
 8. The method ofclaim 7, wherein said reaction product is at least partially reducibleby said molten matrix metal.
 9. The method of claim 8, wherein saidreaction product coats at least a portion of said filler.
 10. The methodof claim 4, wherein said at least one of the infiltration enhancerprecursor and infiltration enhancer is supplied from an external source.11. The method of claim 4, wherein the matrix metal comprises aluminum,the infiltration enhancer precursor comprises calcium and theinfiltrating atmosphere comprises nitrogen.
 12. The method of claim 4,wherein the matrix metal comprises aluminum, the infiltration enhancerprecursor comprises strontium and the infiltrating atmosphere comprisesnitrogen.
 13. The method of claim 4, wherein the matrix metal comprisesaluminum, the infiltration enhancer precursor comprises magnesium andthe infiltrating atmosphere comprises nitrogen.
 14. The method of claim4, wherein the matrix metal comprises aluminum, the infiltrationenhancer precursor comprises zinc, and the infiltrating atmospherecomprises oxygen.
 15. The method of claim 4, wherein said at least oneof said infiltration enhancer precursor and infiltration enhancer isprovided in more than one of said matrix metal, said filler and saidinfiltrating atmosphere.
 16. The method of claim 4, wherein theinfiltration enhancer precursor comprises a material selected from thegroup consisting of magnesium, strontium and calcium.
 17. The method ofclaim 1, further comprising the step of supplying at least one of aninfiltration enhancer precursor and an infiltration enhancer to at leastone of the matrix metal and the filler.
 18. The method of claim 17,wherein said at least one of said infiltration enhancer and saidinfiltration enhancer precursor is provided at a boundary between saidfiller and said matrix metal.
 19. The method of claim 17, wherein saidat least one of said infiltration enhancer precursor and infiltrationenhancer is provided in both of said matrix metal and said filler. 20.The method of any of claims 1, 7 or 8, wherein said ingot substantiallycorresponds, at least in part, to a desired shape of a cavity to beformed in said metal matrix composite.
 21. The method of claim 20,wherein said source of molten matrix metal is substantially infiltratedinto said filler such that a cavity remains in said metal matrixcomposite, said cavity corresponding in shape to said ingot.
 22. Themethod claim 1, further comprising the step of contacting at least aportion of the filler with at least one of an infiltration enhancerprecursor and infiltration enhancer during at least a portion of theperiod of infiltration.
 23. The method of claim 1, wherein the fillercomprises at least one material selected from the group consisting ofpowders, flakes, platelets, microspheres, whiskers, bubbles, fibers,particulates, fiber mats, chopped fibers, spheres, pellets, tubules andrefractory cloths.
 24. The method of claim 1, wherein the filler is oflimited solubility in the molten matrix metal.
 25. The method of claim1, wherein the filler comprises at least one ceramic material.
 26. Themethod of claim 1, wherein an infiltration enhancer precursor is alloyedin said matrix metal.
 27. The method of claim 1, wherein said matrixmetal comprises aluminum and at least one alloying element selected fromthe group consisting of silicon, iron, copper, maganese, chromium, zinc,calcium, magnesium and strontium.
 28. The method of claim 1, wherein thetemperature during spontaneous infiltration is greater than the meltingpoint of the matrix metal, but lower than the volatilization temperatureof the matrix metal and the melting point of the filler.
 29. The methodof claim 1, wherein the matrix metal comprises aluminum and the fillercomprises at least one material selected from the group consisting ofoxides, carbides, borides and nitrides.
 30. A method for making a metalmatrix composite body comprising:providing a shaped ingot of matrixmetal; at least partially surrounding said shape ingot of matrix metalwith a substantially non-reactive filler; providing at least onematerial selected from the group consisting of an infiltration enhancerand an infiltration enhancer precursor to at least one of said shapedingot of matrix metal and said filler; heating at least said shapedingot of matrix metal to render said ingot molten, thereby forming abody of molten matrix metal; communicating an infiltrating atmospherewith at least one of said filler and said shaped ingot of matrix metal;spontaneously infiltrating at least a portion of the filler with moltenmatrix metal; and cooling said molten matrix metal within said filler,thereby forming a metal matrix composite body having a cavity thereinwhich at least partially replicates the configuration of the shapedingot of matrix metal.
 31. A method for making a metal matrix compositebody comprising:providing a shaped ingot of aluminum; at least partiallysurrounding said shaped ingot of aluminum with a filler; providingmagnesium to at least one of said shaped ingot and said filler; heatingat least said shaped ingot of aluminum to render said ingot molten,thereby forming a body of molten aluminum metal; communicating anitrogen-containing atmosphere with at least a portion of at least oneof said filler and said shaped ingot of aluminum; spontaneouslyinfiltrating at least a portion of the filler with molten aluminum; andcooling said molten aluminum within said filler, thereby forming analuminum metal matrix composite body having a cavity therein which atleast partially replicates the configuration of the shaped ingot ofaluminum.
 32. The method of any of claims 1, 6, and 31 wherein thefiller comprises a preform.
 33. The method of any of claims 1, 6, and 31further comprising the step of defining a surface boundary of the fillerwith a barrier, wherein the matrix metal spontaneously infiltrates up tothe barrier.
 34. The method of claim 33, wherein the barrier comprises amaterial selected from the group consisting of carbon, graphite andtitanium diboride.
 35. The method of claim 33, wherein said barrier issubstantially non-wettable by said matrix metal.
 36. The method of claim33, wherein said barrier comprises at least one material which permitscommunication between an infiltrating atmosphere and at least one of thematrix metal, filler, an infiltration enhancer and an infiltrationenhancer precursor.
 37. The method of any of claims 30, 31, or 21,wherein said cavity is at least partially open.
 38. The method of any ofclaims 30, 31, or 21, wherein said ingot is substantially completelysurrounded by said filler and said cavity is substantially completelyclosed.
 39. The method of any of claims 1, 30, 31, 4, 23, or 21, whereinsaid filler is self-supporting at least at some point during the processof spontaneous infiltration.
 40. The method of any of claims 1, 30, 31,4, 23, or 21, wherein said filler is rendered self-supporting by saidheating of said ingot.
 41. The method of any of claims 1, 30, 4, 23, or21, wherein said filler is rendered self-supporting by a bonding agent.42. The method of any of claims 1, 30, 4, 23, or 21, wherein said filleris rendered self-supporting by a chemical reaction with said filler andat least one other species.